Question

how can you say that the field lines formed by the solenoid is as the magnetic field lines are an closed loops ?
answer it please
its urgent

Answer 1

It is believed that magnetic monopoles do not exist [there is a lot of debate going on, one research group even claimed to have detected magnetic monopoles].If magnetic monopoles do not exist, then the divergence of the magnetic has to be zero [to make the flux over any closed surface zero]. This is possible only if magnetic field lines form closed loops [you see there is no other magnetic "charge" where the lines can terminate]. So, inside a solenoid or bar magnet, the field has to be opposite to its direction outside. This can be even tested by placing a magnetic compass inside.